JP7485515B2 - Adhesive layer and adhesive sheet - Google Patents

Description

The present invention relates to an adhesive layer and an adhesive sheet having the adhesive layer.

Generally, adhesives (also called pressure-sensitive adhesives; the same applies below) are in a soft solid (viscoelastic) state at temperatures around room temperature, and have the property of easily adhering to an adherend when pressure is applied. Taking advantage of these properties, adhesives are typically in the form of adhesive sheets containing a layer of the adhesive, and are widely used in a variety of industrial fields, from home appliances to automobiles and office equipment.

Adhesive sheets are also preferably used to bond and fix various optical components in displays such as liquid crystal displays and organic EL displays, and other electronic devices. Patent Document 1 is an example of a technical document related to this type of adhesive sheet.

JP 2018-72837 A

When various optical components are bonded using an adhesive sheet, the adhesive layer is typically placed between two optical components, and the two optical components are bonded by pressing them against each other. At this time, due to the difference in refractive index between the adhesive layer and the optical components, light scattering occurs at the interface between the adhesive layer and the optical components, which causes a problem of reduced light transmittance of the laminate of bonded optical components (for example, a laminate of one optical component, an adhesive layer, and another optical component).

In order to suppress the decrease in light transmittance due to light scattering at the interface between the adhesive layer and the optical member, it is effective to increase the refractive index of the adhesive layer as much as possible and reduce the difference in refractive index between the adhesive layer and the optical member. A commonly known method for increasing the refractive index of a resin is to include a component having a bulky skeleton such as a benzene ring in the constituent components of the resin. However, when such a high refractive index component is included in the adhesive, the properties as an adhesive (e.g., adhesive strength) tend to decrease.

The present invention has been made in consideration of the above circumstances, and aims to provide an adhesive layer that has a high refractive index and high adhesive strength. Another aim of the present invention is to provide an adhesive sheet that includes the above adhesive layer.

According to the present invention, there is provided an adhesive layer containing an adhesive obtained using an adhesive composition containing a base polymer. Here, the refractive index of the adhesive layer is 1.54 or more. The glass transition temperature of the base polymer is 5°C or less. With this configuration, an adhesive layer with high adhesive strength can be easily obtained while still having a high refractive index.

According to a preferred embodiment of the technology disclosed herein, the base polymer is an acrylic polymer. When an acrylic polymer is used as the base polymer, it is easy to obtain a pressure-sensitive adhesive layer that is highly transparent and has improved adhesive strength.

According to another preferred embodiment of the technology disclosed herein, the acrylic polymer is a polymer of monomer components containing one or both of n-butyl acrylate and 2-ethylhexyl acrylate, and the total amount of the n-butyl acrylate and the 2-ethylhexyl acrylate accounts for 20% by weight or more of the total amount of the monomer components. With such a monomer component configuration, it is easy to control the glass transition temperature of the acrylic polymer as the base polymer within a suitable range, and it is easy to improve the adhesive strength of the adhesive layer containing the base polymer.

In addition, in order to impart various properties, organic or inorganic fillers may be added to the adhesive layer in addition to the adhesive. For example, a thermally conductive filler such as a hydrated metal compound may be added to impart thermal conductivity to the adhesive layer. However, the addition of a thermally conductive filler may cause the adhesive layer itself to become cloudy, reducing transparency, due to the influence of light scattering at the interface between the adhesive and the thermally conductive filler contained in the adhesive layer. If an adhesive layer that maintains high light transmittance even when a thermally conductive filler is added is realized, it can simultaneously perform roles such as heat dissipation and heat transfer from the adherend in applications requiring transparency, such as optical applications.

According to another preferred embodiment of the technology disclosed herein, the pressure-sensitive adhesive layer further contains a thermally conductive filler. By containing the thermally conductive filler, the thermal conductivity of the pressure-sensitive adhesive layer is improved. In an embodiment in which the pressure-sensitive adhesive layer is directly attached to an adherend, the pressure-sensitive adhesive layer having such a configuration contributes to efficient heat dissipation, heat transfer, etc. from the adherend.

Here, the refractive index of the thermally conductive filler tends to be relatively high compared to the refractive index of the adhesive commonly used in the field of adhesive sheets, and the large difference in refractive index between the thermally conductive filler and the adhesive in the adhesive layer was one of the factors that reduced the light transmittance of the adhesive layer itself. In this regard, since the refractive index of the adhesive layer disclosed here is relatively high at 1.54 or more, the refractive index difference between the thermally conductive filler and the adhesive contained in the adhesive layer tends to be sufficiently small. Therefore, according to this configuration, the influence of light scattering at the interface between the adhesive and the thermally conductive filler contained in the adhesive layer is suppressed, and the transparency (light transmittance) of the adhesive layer tends to be improved. Therefore, according to the technology disclosed here, it is easy to obtain an adhesive layer with high light transmittance while containing a thermally conductive filler. Furthermore, according to the technology disclosed here, it is easy to obtain an adhesive layer with high adhesive strength. Therefore, according to this configuration, even if the thermally conductive filler is contained in an amount that can exhibit sufficiently high thermal conductivity, it is easy to obtain an adhesive layer with high light transmittance and high adhesive strength.

According to another preferred embodiment of the adhesive layer disclosed herein, the thermally conductive filler contains a hydrated metal compound. Such a thermally conductive filler has excellent thermal conductivity and a refractive index relatively close to that of the adhesive contained in the adhesive layer. Therefore, according to a configuration containing a hydrated metal compound as a thermally conductive filler, even if the adhesive layer contains an amount of thermally conductive filler that can exhibit sufficiently high thermal conductivity, it is easy to obtain an adhesive layer that exhibits excellent light transmittance.

According to another preferred embodiment of the adhesive layer disclosed herein, the refractive index difference between the adhesive layer and the thermally conductive filler is within ±0.04. When the refractive index difference between the adhesive layer and the thermally conductive filler is within the above-mentioned predetermined value, the refractive index difference between the adhesive contained in the adhesive layer and the thermally conductive filler is likely to be within a suitable range. Therefore, according to this configuration, it is easy to obtain an adhesive layer with more favorable improved light transmittance.

According to the technology disclosed herein, an adhesive sheet comprising any one of the adhesive layers disclosed herein is provided. The adhesive layer disclosed herein tends to have a high refractive index while exhibiting excellent adhesive strength. Therefore, with an adhesive sheet comprising such an adhesive layer, when the adherend is a material having a relatively high refractive index such as an optical member, the difference in refractive index between the adhesive layer and the adherend is small, and in a mode in which the adhesive layer is directly attached to the adherend, the decrease in light transmittance at the interface between the adhesive layer and the adherend is easily suppressed.

In addition, when the adhesive layer contains a thermally conductive filler, the adhesive layer disclosed herein is likely to have improved optical transparency and high adhesive strength. An adhesive sheet containing such an adhesive layer is likely to have high transparency and high adhesive strength while being an adhesive sheet that is excellent in heat dissipation and heat transfer from the adherend.

The adhesive sheet according to a preferred embodiment disclosed herein is an adhesive sheet composed of any of the adhesive layers disclosed herein. That is, the adhesive sheet according to this embodiment is a substrate-less adhesive sheet composed of an adhesive layer. An adhesive sheet having such a configuration is not affected by the optical properties of the substrate compared to an adhesive sheet that includes a substrate, and therefore can more effectively exhibit the optical properties of the adhesive layer according to the present invention.

FIG. 1 is a schematic cross-sectional view showing the configuration of a pressure-sensitive adhesive sheet according to one embodiment. FIG. 2 is a schematic cross-sectional view showing the configuration of a pressure-sensitive adhesive sheet according to another embodiment. FIG. 3(a) is a schematic front view of a thermal characteristic evaluation device used for measuring thermal resistance values in the examples, and FIG. 3(b) is a schematic side view of the device shown in FIG. 3(a).

Preferred embodiments of the present invention will be described below. Note that matters necessary for carrying out the present invention other than those specifically mentioned in this specification can be understood by those skilled in the art based on the teachings on carrying out the invention described in this specification and the common general technical knowledge at the time of filing. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the relevant field.
In the following drawings, the same reference numerals may be used to denote components or parts having the same function, and duplicated descriptions may be omitted or simplified. In addition, the embodiments shown in the drawings are schematic in order to clearly explain the present invention, and do not necessarily accurately represent the size or scale of the product actually provided.

<Structural example of adhesive sheet>
The pressure-sensitive adhesive sheet disclosed herein is configured to include a pressure-sensitive adhesive layer. The pressure-sensitive adhesive sheet disclosed herein may be configured from the pressure-sensitive adhesive layer. That is, the pressure-sensitive adhesive sheet disclosed herein may be in the form of a substrateless pressure-sensitive adhesive sheet having a first adhesive surface configured by one surface of the pressure-sensitive adhesive layer and a second adhesive surface configured by the other surface of the pressure-sensitive adhesive layer.

The structure of an adhesive sheet according to one embodiment is shown in FIG. 1. The adhesive sheet 10 is configured as a substrate-less adhesive sheet 10 made of an adhesive layer 12. The adhesive sheet 10 has a first adhesive surface 12A formed by one surface of the adhesive layer 12 and a second adhesive surface 12B formed by the other surface of the adhesive layer 12. The adhesive sheet 10 is used by attaching the first adhesive surface 12A and the second adhesive surface 12B to different locations of another member. The locations to which the first adhesive surface 12A and the second adhesive surface 12B are attached may be respective locations of different members, or may be different locations within a single member. The adhesive sheet 10 before use (i.e., before being attached to an adherend) may be a component of an adhesive sheet with release liner 100 in a form in which the first adhesive surface 12A and the second adhesive surface 12B are protected by release liners 14, 16, at least the side facing the adhesive layer 12 being a release surface, as shown in FIG. 1. As the release liners 14 and 16, for example, a sheet-like substrate (liner substrate) may preferably be used that is configured such that one side is a release surface by providing a release layer made of a release treatment agent on that side. Alternatively, the release liner 16 may be omitted, and a release liner 14 may be used in which both sides are release surfaces, and this may be superimposed on the adhesive sheet 10 and rolled up in a spiral shape to form an adhesive sheet with a release liner in a form (roll form) in which the second adhesive surface 12B is protected by contacting the back surface of the release liner 14.

Alternatively, the adhesive sheet disclosed herein may be in the form of an adhesive sheet with a substrate in which an adhesive layer is laminated on one or both sides of a supporting substrate. Hereinafter, the supporting substrate may be simply referred to as the "substrate".

The structure of the adhesive sheet according to one embodiment is shown in FIG. 2. The adhesive sheet 20 is configured as an adhesive sheet with substrate (double-sided adhesive sheet) 20, which includes a sheet-like support substrate (e.g., a resin film) 22 having a first surface 22A and a second surface 22B, a first adhesive layer 24 fixedly provided on the first surface 22A side, and a second adhesive layer 26 fixedly provided on the second surface 22B side. As shown in FIG. 2, the adhesive sheet 20 before use may be a component of an adhesive sheet with release liner 200 in a form in which the surface (first adhesive surface) 24A of the first adhesive layer 24 and the surface (second adhesive surface) 26A of the second adhesive layer 26 are protected by release liners 28, 29. Alternatively, the release liner 29 may be omitted, and a release liner 28 having both sides as release surfaces may be used, which may be superimposed on the adhesive sheet 20 and rolled up in a spiral shape to configure an adhesive sheet with release liner in a form in which the second adhesive surface 26A is protected by contacting the back surface of the release liner 28 (roll form).

In the adhesive sheet 20 having such a configuration, the material constituting the support substrate 22 is not particularly limited. From the viewpoint of obtaining an adhesive sheet 20 with good light transmission, a transparent resin film can be preferably used as the support substrate 22. Non-limiting examples of resin films include polyolefin films mainly composed of polyolefins such as polypropylene and ethylene-propylene copolymers; polyester films mainly composed of polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate; polyvinyl chloride films mainly composed of polyvinyl chloride; and the like. In one preferred example, a PET film can be preferably used from the viewpoint of transparency.

The concept of adhesive sheet here may include those called adhesive tape, adhesive film, adhesive label, etc. The adhesive sheet may be in roll form or in sheet form, and may be cut, punched, or otherwise processed into an appropriate shape depending on the application or mode of use. In an adhesive sheet in which an adhesive layer is laminated on one or both sides of a support substrate, the adhesive layer is typically formed continuously, but is not limited to this, and may be formed in a regular or random pattern, such as dots or stripes.

<Adhesive Layer>
The pressure-sensitive adhesive sheet disclosed herein includes a pressure-sensitive adhesive layer having a refractive index of 1.54 or more (typically 1.540 or more). In applications where an optical member having a relatively high refractive index is used as an adherend, the pressure-sensitive adhesive layer having a refractive index of 1.54 or more is directly attached to the optical member and used to suppress light scattering at the interface between the pressure-sensitive adhesive layer and the optical member. Therefore, when such a pressure-sensitive adhesive layer is used, the decrease in light transmittance caused by light scattering at the interface between the pressure-sensitive adhesive layer and the adherend optical member can be suppressed.

The refractive index of the adhesive layer is not particularly limited as long as it is 1.54 or more, and may be appropriately set according to the refractive index of the adherend to which the adhesive layer is attached or the refractive index of additives such as fillers that may be added to the adhesive layer. For example, the refractive index of the adhesive layer is preferably 1.541 or more, more preferably 1.542 or more, and even more preferably 1.543 or more (e.g., 1.545 or more). In general, the higher the refractive index of the adhesive layer, the lower the adhesive properties (e.g., adhesive strength) of the adhesive layer tend to be. Therefore, it is meaningful to use the technology disclosed herein to obtain an adhesive layer that has a refractive index equal to or higher than the lower limit described above while exhibiting excellent adhesive strength.

In particular, in an embodiment in which the adhesive layer contains a thermally conductive filler as described below, from the viewpoint of suppressing a decrease in light transmittance due to light scattering or the like at the interface between the thermally conductive filler and other components (mainly the adhesive) in the adhesive layer, it is preferable that the difference in refractive index between the thermally conductive filler and the adhesive in the adhesive layer is small. Thermally conductive fillers tend to have a higher refractive index than adhesives commonly used in the field of adhesive sheets, so increasing the refractive index of the adhesive and the adhesive layer containing the adhesive is preferable from the viewpoint of improving the light transmittance of the adhesive layer itself.

From this viewpoint, particularly in an embodiment in which the adhesive layer contains a thermally conductive filler, the refractive index of the adhesive layer is preferably 1.542 or more. When aluminum hydroxide is used as the thermally conductive filler, the refractive index of the adhesive layer is preferably 1.545 or more, more preferably 1.548 or more, and even more preferably 1.550 or more. When magnesium hydroxide is used as the thermally conductive filler, the refractive index of the adhesive layer is preferably 1.542 or more, more preferably 1.545 or more, and even more preferably 1.547 or more.

The upper limit of the refractive index of the adhesive layer is not particularly limited. In terms of balancing with other adhesive properties (e.g., adhesive strength), it is usually appropriate for the refractive index of the adhesive layer to be 1.590 or less. When the adhesive layer contains a thermally conductive filler, in terms of reducing the difference in refractive index between the thermally conductive filler and the adhesive in the adhesive layer, the refractive index of the adhesive layer is preferably 1.585 or less, more preferably 1.580 or less. When aluminum hydroxide is used as the thermally conductive filler, the refractive index of the adhesive layer is preferably 1.575 or less, more preferably 1.570 or less, and even more preferably 1.565 or less. When magnesium hydroxide is used as the thermally conductive filler, the refractive index of the adhesive layer is preferably 1.565 or less, more preferably 1.560 or less, and even more preferably 1.555 or less.

In this specification, the refractive index of the adhesive layer can be measured using a commercially available Abbe refractometer (e.g., model "DR-M2", manufactured by ATAGO). More specifically, the refractive index of the adhesive layer can be measured by the method described in the examples below. The same applies to the refractive index of the thermally conductive filler described below.

In the technology disclosed herein, the adhesive contained in the adhesive layer is not particularly limited. The adhesive may be, for example, an adhesive containing one or more of various polymers such as acrylic polymers, rubber polymers, polyester polymers, urethane polymers, polyether polymers, silicone polymers, polyamide polymers, and fluorine polymers as a base polymer (i.e., a component that accounts for 50% by weight or more of the polymer components). The adhesive layer in the technology disclosed herein may be formed from an adhesive composition containing such a base polymer. The form of the adhesive composition is not particularly limited, and may be an adhesive composition in various forms such as a water-dispersed type, a solvent type, a hot melt type, or an active energy ray curable type (e.g., a photocurable type).

In this specification, "active energy rays" refers to energy rays that have the energy to cause chemical reactions such as polymerization reactions, crosslinking reactions, and decomposition of initiators. Examples of active energy rays include light such as ultraviolet (UV) rays, visible light, and infrared rays, as well as radiation such as alpha rays, beta rays, gamma rays, electron beams, neutron beams, and X-rays.

(Base polymer)
In the technology disclosed herein, the base polymer has a glass transition temperature (Tg) of 5° C. or less. A pressure-sensitive adhesive containing a base polymer with such a Tg is suitable for forming a pressure-sensitive adhesive layer with good unevenness-following properties, and tends to have improved adhesive performance (adhesive strength, etc.). From this perspective, the Tg of the base polymer is more preferably 3° C. or less, and may be less than 0° C. In some embodiments, the Tg of the base polymer may be, for example, less than −3° C. or less than −5° C. Although the lower limit of the Tg of the base polymer is not particularly limited, a base polymer having a Tg of −40° C. or more may be preferably used from the viewpoint of balancing with the improvement of the refractive index of the pressure-sensitive adhesive layer. In some embodiments, the Tg of the base polymer may be, for example, −20° C. or more, −15° C. or more, or −10° C. or more.

Here, the Tg of the base polymer refers to a nominal value described in literature, catalogs, etc., or a Tg calculated by Fox's formula based on the composition of the monomer components used in the preparation of the base polymer. The Fox formula is a relational expression between the Tg of a copolymer and the glass transition temperature Tgi of a homopolymer obtained by homopolymerizing each of the monomers constituting the copolymer, as shown below.
1/Tg=Σ(Wi/Tgi)
In the above Fox formula, Tg represents the glass transition temperature (unit: K) of the copolymer, Wi represents the weight fraction of monomer i in the copolymer (copolymerization ratio on a weight basis), and Tgi represents the glass transition temperature (unit: K) of a homopolymer of monomer i. When the base polymer is a homopolymer, the Tg of the homopolymer and the Tg of the base polymer are the same.

The glass transition temperature of the homopolymer used to calculate Tg should be a value listed in a publicly available document. Specifically, values are listed in "Polymer Handbook" (3rd Edition, John Wiley & Sons, Inc., 1989). For monomers for which multiple values are listed in the Polymer Handbook, the highest value is used. For the glass transition temperature of homopolymers of monomers not listed in the Polymer Handbook, the value obtained by the measurement method described in JP-A-2007-51271 can be used.

Although not particularly limited, the weight average molecular weight (Mw) of the base polymer may be, for example, about 5×10 ⁴ or more. A base polymer with such Mw is likely to produce a pressure-sensitive adhesive exhibiting good cohesiveness. In some embodiments, the Mw of the base polymer may be, for example, 10×10 ⁴ or more, 20×10 ⁴ or more, or 30×10 ⁴ or more. In addition, it is usually appropriate that the Mw of the base polymer is about 500×10 ⁴ or less. A base polymer with such Mw is suitable for forming a pressure-sensitive adhesive layer with good unevenness-following ability.

The Mw of the base polymer can be determined as a polystyrene-equivalent value by gel permeation chromatography (GPC). The GPC measurement can be performed, for example, using a GPC device, HLC-8220GPC, manufactured by Tosoh Corporation, under the following conditions.
[GPC Measurement]
Sample concentration: 0.2 wt% (tetrahydrofuran (THF) solution)
Sample injection volume: 10 μl
Eluent: THF Flow rate: 0.6 ml/min
Measurement temperature: 40°C
·column:
Sample column: TSKguardcolumn SuperHZ-H (1 column) + TSKgel SuperHZM-H (2 columns)
Reference column: TSKgel SuperH-RC (1 column)
Detector: differential refractometer (RI)

In the technology disclosed herein, the base polymer is preferably an acrylic polymer.
In this specification, the term "acrylic polymer" refers to a polymer containing monomer units derived from (meth)acrylic monomers in the polymer structure, and typically refers to a polymer containing monomer units derived from (meth)acrylic monomers in a proportion of more than 50% by weight.
In addition, the (meth)acrylic monomer refers to a monomer having at least one (meth)acryloyl group in one molecule. Here, the term "(meth)acryloyl group" refers to an acryloyl group and a methacryloyl group in a comprehensive sense. Therefore, the concept of the (meth)acrylic monomer here can include both a monomer having an acryloyl group (acrylic monomer) and a monomer having a methacryloyl group (methacrylic monomer). Similarly, in this specification, "(meth)acrylic acid" refers to acrylic acid and methacrylic acid, and "(meth)acrylate" refers to acrylate and methacrylate in a comprehensive sense.

In some embodiments, the acrylic polymer is a polymer containing a monomer unit derived from a (meth)acrylic acid alkyl ester. As the (meth)acrylic acid alkyl ester, a (meth)acrylic acid alkyl ester having a linear or branched alkyl group having 1 to 20 carbon atoms (i.e., C _1-20 ) can be preferably used. Since it is easy to balance the properties, the proportion of the (meth)acrylic acid C _1-20 alkyl ester in the total amount of the monomer components may be, for example, 10% by weight or more, 15% by weight or more, or 20% by weight or more in some embodiments. For the same reason, the proportion of the (meth)acrylic acid C _1-20 alkyl ester in the total amount of the monomer components may be, for example, 50% by weight or less, 45% by weight or less, or 40% by weight or less.

Non-limiting specific examples of the (meth)acrylic acid C _1-20 alkyl ester include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and (meth)acrylic acid. Examples of the acrylates include isooctyl (meth)acrylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, nonadecyl (meth)acrylate, and eicosyl (meth)acrylate.

Of these, it is preferable to use at least a (meth)acrylic acid C _1-18 alkyl ester, and it is more preferable to use at least a (meth)acrylic acid C _1-14 alkyl ester. In some embodiments, the acrylic polymer may contain at least one selected from a (meth)acrylic acid C _4-12 alkyl ester (preferably an acrylic acid C _4-10 alkyl ester) as a monomer unit. For example, an acrylic polymer containing one or both of n-butyl acrylate (BA) and 2-ethylhexyl acrylate (2EHA) is preferred. Examples of other (meth)acrylic acid C _1-18 alkyl esters that can be preferably used include methyl acrylate, methyl methacrylate (MMA), n-butyl methacrylate (BMA), 2-ethylhexyl methacrylate (2EHMA), isostearyl acrylate (ISTA), and the like.

In a preferred embodiment of the technology disclosed herein, the total content of the n-butyl acrylate (BA) and the 2-ethylhexyl acrylate (2EHA) in the total amount of monomer components is 15% by weight or more. With this configuration, the glass transition temperature of the acrylic polymer is likely to be 5° C. or less, and an adhesive layer containing an acrylic polymer having such a relatively low glass transition temperature as a base polymer is likely to have improved unevenness-following ability and improved adhesive strength. From this perspective, the total content of BA and 2EHA is more preferably 20% by weight or more, and even more preferably 23% by weight or more, based on the total amount of monomer components. In a preferred embodiment, the total content of BA and 2EHA may be 25% by weight or more, or 28% by weight or more, based on the total amount of monomer components. From the perspective of balancing with other properties, the total amount of BA and 2EHA in the total amount of monomer components may be, for example, 50% by weight or less, 45% by weight or less, or 40% by weight or less.

The monomer units constituting the acrylic polymer may contain other monomers (hereinafter also referred to as copolymerizable monomers) other than (meth)acrylic acid alkyl esters that are copolymerizable with (meth)acrylic acid alkyl esters. As the copolymerizable monomer, an acrylic monomer that is a high refractive index monomer can be suitably adopted. Such a high refractive index monomer is likely to improve the refractive index of a polymer of a monomer component containing the high refractive index monomer. Here, in this specification, the "high refractive index monomer" refers to a monomer whose homopolymer has a refractive index of approximately 1.50 or more. Examples of acrylic high refractive index monomers include (meth)acrylates having an aromatic ring, sulfur-containing (meth)acrylates, halogenated (meth)acrylates, etc. These can be used alone or in combination of two or more. Among them, (meth)acrylates having an aromatic ring can be preferably used.

Non-limiting examples of (meth)acrylates having an aromatic ring include benzyl (meth)acrylate, naphthyl (meth)acrylate, phenoxyethyl (meth)acrylate, phenoxybutyl (meth)acrylate, optionally ethoxylated phenylphenol (meth)acrylate, and fluorene-based (meth)acrylate. Among these, optionally ethoxylated phenylphenol (meth)acrylate and benzyl (meth)acrylate are preferred, and optionally ethoxylated phenylphenol acrylate (e.g., ethoxylated o-phenylphenol acrylate) and benzyl acrylate are more preferred.

Here, the fluorene-based (meth)acrylate is a compound (monomer) having a fluorene skeleton and a (meth)acryloyl group in the molecule, and is preferably a compound having a structure in which a (meth)acryloyl group is bonded to the fluorene skeleton directly or via an oxyalkylene chain (monooxyalkylene chain or polyoxyalkylene chain). Among such fluorene-based (meth)acrylates, so-called polyfunctional fluorene-based (meth)acrylates in which the number of (meth)acryloyl groups (including via an oxyalkylene chain) bonded to the fluorene skeleton is two or more are preferred. Specific examples of the fluorene-based (meth)acrylates include products manufactured by Osaka Gas Chemicals Co., Ltd. under the names "Oxol EA-0200", "EA-0500", "EA-1000", etc. In one aspect of the technology disclosed herein, the fluorene-based (meth)acrylate may not be used from the viewpoint of improving the adhesive strength of the adhesive layer.

Suitable examples of the sulfur-containing (meth)acrylate include 1,2-bis(meth)acryloylthioethane, 1,3-bis(meth)acryloylthiopropane, 1,4-bis(meth)acryloylthiobutane, 1,2-bis(meth)acryloylmethylthiobenzene, and 1,3-bis(meth)acryloylmethylthiobenzene.
Suitable examples of halogenated (meth)acrylates include 6-(4,6-dibromo-2-isopropylphenoxy)-1-hexyl acrylate, 6-(4,6-dibromo-2-s-butylphenoxy)-1-hexyl acrylate, 2,6-dibromo-4-nonylphenyl acrylate, and 2,6-dibromo-4-dodecylphenyl acrylate.

From the viewpoint of being able to suitably adjust the refractive index of the base polymer and therefore the refractive index of the adhesive layer, the proportion of the acrylic high refractive index monomer in the total amount of the monomer components of the base polymer is preferably 50% by weight or more. The content of the acrylic high refractive index monomer in the monomer components is more preferably 55% by weight or more, and may be 60% by weight or more. In some embodiments, the proportion of the acrylic high refractive index monomer in the total amount of the monomer components is preferably 70% by weight or more, and may be 80% by weight or more. Since it is easy to balance other properties such as adhesion, in some embodiments, the proportion of the acrylic high refractive index monomer in the total amount of the monomer components may be 80% by weight or less, 75% by weight or less, 70% by weight or less, or 65% by weight or less. In addition, when the monomer components of the base polymer contain two or more types of acrylic high refractive index monomers, the content of the acrylic high refractive index monomer in the monomer components refers to the total amount of the two or more types of acrylic high refractive index monomers.

The monomer units constituting the acrylic polymer may contain other monomers (hereinafter also referred to as copolymerizable monomers) other than (meth)acrylic acid alkyl esters and high refractive index acrylic monomers that are copolymerizable with (meth)acrylic acid alkyl esters or high refractive index acrylic monomers, as necessary. As the copolymerizable monomers, monomers having polar groups (e.g., carboxy groups, hydroxyl groups, amide groups, etc.) can be suitably used. Monomers having polar groups can be useful for introducing crosslinking points into the acrylic polymer or for increasing the cohesive force of the acrylic polymer. The copolymerizable monomers can be used alone or in combination of two or more.

Non-limiting examples of copolymerizable monomers include the following:
Carboxy group-containing monomers: for example, acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid, crotonic acid, isocrotonic acid, and the like.
Acid anhydride group-containing monomers: for example, maleic anhydride, itaconic anhydride.
Hydroxyl group-containing monomers: for example, hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, 10-hydroxydecyl (meth)acrylate, 12-hydroxylauryl (meth)acrylate, and (4-hydroxymethylcyclohexyl)methyl (meth)acrylate.
Monomers containing a sulfonic acid group or a phosphoric acid group: for example, styrenesulfonic acid, allylsulfonic acid, sodium vinylsulfonate, 2-(meth)acrylamido-2-methylpropanesulfonic acid, (meth)acrylamidopropanesulfonic acid, sulfopropyl (meth)acrylate, (meth)acryloyloxynaphthalenesulfonic acid, 2-hydroxyethylacryloylphosphate, and the like.
Epoxy group-containing monomers: for example, epoxy group-containing acrylates such as glycidyl (meth)acrylate and 2-ethyl glycidyl ether (meth)acrylate, allyl glycidyl ether, and glycidyl ether (meth)acrylate.
Cyano group-containing monomers: for example, acrylonitrile, methacrylonitrile, etc.
Isocyanate group-containing monomers: for example, 2-isocyanatoethyl (meth)acrylate.
Amide group-containing monomers: for example, (meth)acrylamide; N,N-dialkyl(meth)acrylamides such as N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N,N-dipropyl(meth)acrylamide, N,N-diisopropyl(meth)acrylamide, N,N-di(n-butyl)(meth)acrylamide, and N,N-di(t-butyl)(meth)acrylamide; N-alkyl(meth)acrylamides such as N-ethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-butyl(meth)acrylamide, and N-n-butyl(meth)acrylamide; N-vinyl carboxylic acid amides such as N-vinylacetamide; monomers having a hydroxyl group and an amide group, for example, N-(2-hydroxyethyl)(meth)acrylamide; N-hydroxyalkyl(meth)acrylamide, such as N-(2-hydroxypropyl)(meth)acrylamide, N-(1-hydroxypropyl)(meth)acrylamide, N-(3-hydroxypropyl)(meth)acrylamide, N-(2-hydroxybutyl)(meth)acrylamide, N-(3-hydroxybutyl)(meth)acrylamide, and N-(4-hydroxybutyl)(meth)acrylamide; monomers having an alkoxy group and an amide group, such as N-alkoxyalkyl(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-methoxyethyl(meth)acrylamide, and N-butoxymethyl(meth)acrylamide; and others, such as N,N-dimethylaminopropyl(meth)acrylamide and N-(meth)acryloylmorpholine.
Monomers having a nitrogen atom-containing ring: for example, N-vinyl-2-pyrrolidone, N-methylvinylpyrrolidone, N-vinylpyridine, N-vinylpiperidone, N-vinylpyrimidine, N-vinylpiperazine, N-vinylpyrazine, N-vinylpyrrole, N-vinylimidazole, N-vinyloxazole, N-(meth)acryloyl-2-pyrrolidone, N-(meth)acryloylpiperidine, N-(meth)acryloylpyrrolidine, N-vinylmorpholine, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, N-vinyl-3,5-morpholinedione, N-vinylpyrazole, N-vinylisoxazole, N-vinylthiazole, N-vinylisothiazole, N-vinylpyridazine, etc. (for example, lactams such as N-vinyl-2-caprolactam).
Monomers having a succinimide skeleton: for example, N-(meth)acryloyloxymethylene succinimide, N-(meth)acryloyl-6-oxyhexamethylene succinimide, N-(meth)acryloyl-8-oxyhexamethylene succinimide, and the like.
Maleimides: for example, N-cyclohexylmaleimide, N-isopropylmaleimide, N-laurylmaleimide, N-phenylmaleimide, and the like.
Itaconimides: for example, N-methylitaconimide, N-ethylitaconimide, N-butylitaconimide, N-octylitaconimide, N-2-ethylhexylitaconimide, N-cyclohexylitaconimide, N-laurylitaconimide, and the like.
Aminoalkyl (meth)acrylates: for example, aminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, and t-butylaminoethyl (meth)acrylate.
Alkoxy group-containing monomers: for example, alkoxyalkyl (meth)acrylates such as 2-methoxyethyl (meth)acrylate, 3-methoxypropyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, propoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, and ethoxypropyl (meth)acrylate; alkoxyalkylene (meth)acrylates such as methoxyethylene glycol (meth)acrylate and methoxypolypropylene glycol (meth)acrylate.
Vinyl esters: for example, vinyl acetate, vinyl propionate, etc.
Vinyl ethers: for example, vinyl alkyl ethers such as methyl vinyl ether and ethyl vinyl ether.
Aromatic vinyl compounds: for example, styrene, α-methylstyrene, vinyltoluene, etc.
Olefins: For example, ethylene, butadiene, isoprene, isobutylene, etc.
(Meth)acrylic acid esters having an alicyclic hydrocarbon group: for example, cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and the like.
(Meth)acrylic acid esters having an aromatic hydrocarbon group: for example, phenyl (meth)acrylate, phenoxyethyl (meth)acrylate, benzyl (meth)acrylate, and the like.
Other examples include heterocycle-containing (meth)acrylates such as tetrahydrofurfuryl (meth)acrylate, halogen atom-containing (meth)acrylates such as vinyl chloride and fluorine atom-containing (meth)acrylates, silicon atom-containing (meth)acrylates such as silicone (meth)acrylate, and (meth)acrylic acid esters obtained from alcohols derived from terpene compounds.

In some embodiments, the copolymerizable monomer that can be preferably used is at least one monomer selected from the group consisting of N-vinyl cyclic amide represented by the following general formula (M1) and a hydroxyl group-containing monomer (a monomer having a hydroxyl group and another functional group, for example, a monomer containing a hydroxyl group and an amide group).

ここで、上記一般式（Ｍ１）中のＲ^１は、２価の有機基である。

Here, ^R1 in the above general formula (M1) is a divalent organic group.

Specific examples of N-vinyl cyclic amides include N-vinyl-2-pyrrolidone, N-vinyl-2-piperidone, N-vinyl-3-morpholinone, N-vinyl-2-caprolactam, N-vinyl-1,3-oxazin-2-one, and N-vinyl-3,5-morpholinedione. N-vinyl-2-pyrrolidone and N-vinyl-2-caprolactam are particularly preferred.

Specific examples of hydroxyl group-containing monomers that can be suitably used include 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, and N-(2-hydroxyethyl)(meth)acrylamide. Among these, preferred examples include 2-hydroxyethyl acrylate (HEA), 4-hydroxybutyl acrylate (4HBA), and N-(2-hydroxyethyl)acrylamide (HEAA).

When using the above-mentioned copolymerizable monomers, the amount used is not particularly limited, but it is usually appropriate to make it 0.01% by weight or more of the total amount of the monomer components. From the viewpoint of better exerting the effect of using the copolymerizable monomers, the amount of the copolymerizable monomer used may be 0.1% by weight or more of the total amount of the monomer components, or may be 1% by weight or more. In addition, the amount of the copolymerizable monomer used may be 50% by weight or less of the total amount of the monomer components, and preferably 40% by weight or less. This can improve the unevenness-following ability.

The method for obtaining the acrylic polymer is not particularly limited, and various polymerization methods known as methods for synthesizing acrylic polymers, such as solution polymerization, emulsion polymerization, bulk polymerization, suspension polymerization, and photopolymerization, can be appropriately used. In some embodiments, the solution polymerization method or photopolymerization method can be preferably used.

The initiator used for the polymerization can be appropriately selected from conventionally known thermal polymerization initiators, photopolymerization initiators, etc., depending on the polymerization method. The polymerization initiators can be used alone or in combination of two or more.
Examples of the thermal polymerization initiator include azo-based polymerization initiators, persulfates, peroxide-based polymerization initiators, redox-based polymerization initiators, etc. The amount of the thermal polymerization initiator used is not particularly limited, but may be, for example, within the range of 0.01 to 5 parts by weight, preferably 0.05 to 3 parts by weight, per 100 parts by weight of the monomer components used in the preparation of the acrylic polymer.

The photopolymerization initiator is not particularly limited, but for example, a benzoin ether-based photopolymerization initiator, an acetophenone-based photopolymerization initiator, an α-ketol-based photopolymerization initiator, an aromatic sulfonyl chloride-based photopolymerization initiator, a photoactive oxime-based photopolymerization initiator, a benzoin-based photopolymerization initiator, a benzyl-based photopolymerization initiator, a benzophenone-based photopolymerization initiator, a ketal-based photopolymerization initiator, a thioxanthone-based photopolymerization initiator, an acylphosphine oxide-based photopolymerization initiator, etc. can be used. The amount of the photopolymerization initiator used is not particularly limited, but for example, it can be in the range of 0.01 to 5 parts by weight, preferably 0.05 to 3 parts by weight, per 100 parts by weight of the monomer components used in the preparation of the acrylic polymer.

In some embodiments, the acrylic polymer may be included in the adhesive composition for forming the adhesive layer in the form of a partial polymer (acrylic polymer syrup) obtained by irradiating a mixture of the monomer components as described above with a polymerization initiator with ultraviolet light to polymerize a part of the monomer components. The adhesive composition containing such an acrylic polymer syrup can be applied to a predetermined substrate and irradiated with ultraviolet light to complete the polymerization. That is, the acrylic polymer syrup can be understood as a precursor of the acrylic polymer. The adhesive layer disclosed herein may be formed, for example, using an adhesive composition containing an acrylic polymer as a base polymer in the form of the acrylic polymer syrup and, if necessary, an appropriate amount of a polyfunctional monomer as described below.

The refractive index of the base polymer is not particularly limited. From the viewpoint of improving the refractive index of the adhesive layer, it is preferable to use a base polymer with a high refractive index. From this viewpoint, the refractive index of the base polymer is preferably 1.540 or more, more preferably 1.541 or more, even more preferably 1.542 or more, and particularly preferably 1.543 or more (for example, 1.545 or more). In an embodiment in which the adhesive layer contains a thermally conductive filler in addition to the base polymer, the refractive index of the base polymer is preferably 1.542 or more. When aluminum hydroxide is used as the thermally conductive filler, the refractive index of the base polymer is preferably 1.545 or more, more preferably 1.548 or more, and even more preferably 1.550 or more. When magnesium hydroxide is used as the thermally conductive filler, the refractive index of the base polymer is preferably 1.542 or more, more preferably 1.545 or more, and even more preferably 1.547 or more. When a base polymer having such a refractive index is used, the difference in refractive index between the base polymer and the thermally conductive filler tends to be small, so that an adhesive layer with excellent transparency is easily realized.

The upper limit of the refractive index of the base polymer is not particularly limited. In terms of balancing with other adhesive properties (e.g., adhesive strength), it is usually appropriate for the refractive index of the base polymer to be 1.590 or less. When the adhesive layer contains a thermally conductive filler in addition to the base polymer, in terms of reducing the difference in refractive index between the base polymer and the thermally conductive filler, the refractive index of the base polymer is preferably 1.585 or less, more preferably 1.580 or less. When aluminum hydroxide is used as the thermally conductive filler, the refractive index of the base polymer is preferably 1.575 or less, more preferably 1.570 or less, and even more preferably 1.565 or less. When magnesium hydroxide is used as the thermally conductive filler, the refractive index of the base polymer is preferably 1.565 or less, more preferably 1.560 or less, and even more preferably 1.555 or less.

(Crosslinking Agent)
In the pressure-sensitive adhesive layer, a crosslinking agent may be used as necessary for the purpose of adjusting the cohesive force, etc. As the crosslinking agent, a crosslinking agent known in the field of pressure-sensitive adhesives may be used, and examples thereof include epoxy-based crosslinking agents, isocyanate-based crosslinking agents, silicone-based crosslinking agents, oxazoline-based crosslinking agents, aziridine-based crosslinking agents, silane-based crosslinking agents, alkyl etherified melamine-based crosslinking agents, and metal chelate-based crosslinking agents. In particular, isocyanate-based crosslinking agents, epoxy-based crosslinking agents, and metal chelate-based crosslinking agents are preferably used. The crosslinking agents may be used alone or in combination of two or more.

When a crosslinking agent is used, the amount used is not particularly limited, and can be, for example, an amount exceeding 0 parts by weight per 100 parts by weight of the base polymer. In addition, the amount of the crosslinking agent used can be, for example, 0.01 parts by weight or more per 100 parts by weight of the base polymer, and is preferably 0.05 parts by weight or more. By increasing the amount of the crosslinking agent used, a higher cohesive strength tends to be obtained. In some embodiments, the amount of the crosslinking agent used per 100 parts by weight of the base polymer may be 0.1 parts by weight or more, 0.5 parts by weight or more, or 1 part by weight or more. On the other hand, from the viewpoint of avoiding a decrease in unevenness-following ability due to an excessive increase in cohesive strength, the amount of the crosslinking agent used per 100 parts by weight of the base polymer is usually appropriate to be 15 parts by weight or less, may be 10 parts by weight or less, or may be 5 parts by weight or less. The technology disclosed herein can be suitably implemented even in a form in which a crosslinking agent is not used.

A crosslinking catalyst may be used to more effectively proceed with any of the crosslinking reactions described above. As the crosslinking catalyst, for example, a tin-based catalyst (particularly dioctyltin dilaurate) can be preferably used. There are no particular restrictions on the amount of the crosslinking catalyst used, but it can be, for example, about 0.0001 to 1 part by weight per 100 parts by weight of the base polymer.

A polyfunctional monomer may be used in the adhesive layer as necessary. The polyfunctional monomer may be used in place of the crosslinking agent as described above or in combination with the crosslinking agent, and may be useful for purposes such as adjusting the cohesive strength. For example, the polyfunctional monomer may be preferably used in the adhesive layer formed from the photocurable adhesive composition.
Examples of the polyfunctional monomer include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, ethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, 1,12-dodecanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, allyl (meth)acrylate, vinyl (meth)acrylate, divinylbenzene, epoxy acrylate, polyester acrylate, urethane acrylate, butyl diol (meth)acrylate, and hexyl diol di(meth)acrylate. Among these, trimethylolpropane tri(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and dipentaerythritol hexa(meth)acrylate can be preferably used. The polyfunctional monomers can be used alone or in combination of two or more.

The amount of the polyfunctional monomer used varies depending on the molecular weight and the number of functional groups, but is usually in the range of about 0.01 to 3.0 parts by weight per 100 parts by weight of the base polymer. In some embodiments, the amount of the polyfunctional monomer used may be, for example, 0.02 parts by weight or more, or 0.03 parts by weight or more, per 100 parts by weight of the base polymer. By increasing the amount of the polyfunctional monomer used, there is a tendency for higher cohesive strength to be obtained. On the other hand, from the viewpoint of avoiding a decrease in unevenness-following ability due to an excessive increase in cohesive strength, the amount of the polyfunctional monomer used may be 2.0 parts by weight or less, 1.0 parts by weight or less, or 0.5 parts by weight or less, per 100 parts by weight of the base polymer.

(Tackifier resin)
The adhesive layer may contain a tackifier resin as required. The tackifier resin is not particularly limited, but may be, for example, a rosin-based tackifier resin, a terpene-based tackifier resin, a phenol-based tackifier resin, a hydrocarbon-based tackifier resin, a ketone-based tackifier resin, a polyamide-based tackifier resin, an epoxy-based tackifier resin, or an elastomer-based tackifier resin. The tackifier resin may be used alone or in combination of two or more.

As the tackifier resin, one having a softening point (softening temperature) of about 80°C or higher (preferably about 100°C or higher, for example about 120°C or higher) can be preferably used. The upper limit of the softening point is not particularly limited, and can be, for example, about 200°C or lower (typically 180°C or lower). The softening point of the tackifier resin can be measured based on the softening point test method (ring and ball method) specified in JIS K2207.

When a tackifier resin is used, its content is not particularly limited, and can be set so that appropriate adhesive performance is exhibited depending on the purpose and application. The content of the tackifier resin (when two or more types of tackifier resins are included, the total amount thereof) relative to 100 parts by weight of the base polymer may be, for example, 5 parts by weight or more, or 10 parts by weight or more. On the other hand, from the viewpoint of improving the unevenness-following ability, in some embodiments, the content of the tackifier resin is appropriately 100 parts by weight or less relative to 100 parts by weight of the base polymer, and may be 50 parts by weight or less, or may be 25 parts by weight or less. Alternatively, no tackifier resin may be used.

(Filler)
In a preferred embodiment of the technology disclosed herein, the pressure-sensitive adhesive layer contains a filler. The filler is not particularly limited, and for example, a particulate or fibrous filler can be used. The filler can be used alone or in combination of two or more types.

Examples of materials constituting the filler include metals such as copper, silver, gold, platinum, nickel, aluminum, chromium, iron, and stainless steel; metal oxides such as aluminum oxide, silicon oxide (typically silicon dioxide), titanium oxide, zirconium oxide, zinc oxide, tin oxide, antimony acid-doped tin oxide, copper oxide, and nickel oxide; aluminum hydroxide [Al ₂ O _{3.3H 2} O; or Al(OH) ₃ ], boehmite [Al ₂ O _{3.H 2} O; or AlOOH], magnesium hydroxide [MgO.H ₂ O; or Mg(OH) ₂ ], calcium hydroxide [CaO.H ₂ O; or Ca(OH) ₂ ], zinc hydroxide [Zn(OH) ₂ ], silicic acid [H ₄ SiO ₄ ; or H ₂ SiO ₃ ; or H ₂ Si ₂ O ₅ ], and iron hydroxide [Fe ₂ O _{3.H 2} O or 2FeO(OH)], copper hydroxide [Cu(OH) ₂ ], barium hydroxide [BaO.H ₂ O; or BaO.9H ₂ O], zirconium oxide hydrate [ZrO.nH ₂ O], tin oxide hydrate [SnO.H ₂ O], basic magnesium carbonate [3MgCO _3.Mg (OH) _2.3H 2 O], hydrotalcite [6MgO.Al ₂ O _{3.H 2} O], dawsonite [Na ₂ CO 3.Al ₂ O 3.nH ₂ O], borax [Na _{2 O.B 2} O _{5.5H 2} O], zinc borate [2ZnO.3B ₂ O 5.3.5H ₂ O], tungsten oxide [Titanium dioxide] _{, zinc} oxide [ _... O] and the like; carbides such as silicon carbide, boron carbide, nitrogen carbide, calcium carbide; nitrides such as aluminum nitride, silicon nitride, boron nitride, gallium nitride; carbonates such as calcium carbonate; titanates such as barium titanate and potassium titanate; carbon-based materials such as carbon black, carbon tubes (typically carbon nanotubes), carbon fibers, diamonds; glass; and other inorganic materials; polymers such as polystyrene, acrylic resins (e.g., polymethyl methacrylate), phenolic resins, benzoguanamine resins, urea resins, silicone resins, polyesters, polyurethanes, polyethylene, polypropylene, polyamides (e.g., nylon, etc.), polyimides, and polyvinylidene chloride; and the like. Alternatively, natural raw material particles such as volcanic sirasu, clay, and sand may be used. In addition, various synthetic fiber materials and natural fiber materials can be used as the fibrous filler.

Particulate fillers are preferably used because the smoothness of the surface of the adhesive layer is not easily impaired even when the content in the adhesive layer is relatively high. The shape of the particles is not particularly limited, and may be bulk, needle, plate, or layer. Bulk shapes include, for example, spherical, rectangular, crushed, or irregular shapes thereof. The structure of the particles is not particularly limited, and may be, for example, a dense structure, a porous structure, a hollow structure, or the like.

When using a photocurable (e.g., ultraviolet-curable) adhesive composition, it is preferable to use a filler made of an inorganic material from the viewpoint of the photocurability (polymerization reactivity) of the adhesive composition.

In the technology disclosed herein, the adhesive layer preferably contains a thermally conductive filler. As the thermally conductive filler, a filler made of an inorganic material can be preferably used. Suitable examples of the thermally conductive filler include fillers with a dense structure made of a hydrated metal compound, a metal oxide, a metal, etc. An adhesive layer containing a thermally conductive filler tends to have improved thermal conductivity.

In some embodiments, a thermally conductive filler composed of a hydrated metal compound may be preferably used. The hydrated metal compound generally has a decomposition onset temperature in the range of about 150 to 500° C. and is a compound represented by the general formula M _x O _{yn.nH 2} O (M is a metal atom, x and y are integers of 1 or more determined by the valence of the metal, and n is the number of water of crystallization) or a double salt containing the compound. Suitable examples of the hydrated metal compound include aluminum hydroxide and magnesium hydroxide.

Hydrated metal compounds are commercially available. Examples of commercially available aluminum hydroxide include "Hidilite H-100-ME" (average primary particle size 75 μm) (manufactured by Showa Denko K.K.), "Hidilite H-10" (average primary particle size 55 μm) (manufactured by Showa Denko K.K.), "Hidilite H-32" (average primary particle size 8 μm) (manufactured by Showa Denko K.K.), "Hidilite H-31" (average primary particle size 20 μm) (manufactured by Showa Denko K.K.), "Hidilite H-42" (average primary particle size 1 μm) (manufactured by Showa Denko K.K.), and "B103ST" (average primary particle size 7 μm) (manufactured by Nippon Light Metal Co., Ltd.). Examples of commercially available magnesium hydroxide include "KISUMA 5A" (average primary particle size 1 μm) (manufactured by Kyowa Chemical Industry Co., Ltd.).

Examples of commercially available thermally conductive fillers other than hydrated metal compounds include boron nitride, such as "HP-40" (manufactured by Mizushima Ferroalloys) and "PT620" (manufactured by Momentive), aluminum oxide, such as "AS-50" (manufactured by Showa Denko K.K.) and "AS-10" (manufactured by Showa Denko K.K.); antimonate-doped tin, such as "SN-100S" (manufactured by Ishihara Sangyo Kaisha), "SN-100P" (manufactured by Ishihara Sangyo Kaisha), and "SN-100D (water-dispersed product)" (manufactured by Ishihara Sangyo Kaisha); titanium oxide, such as "TTO series" (manufactured by Ishihara Sangyo Kaisha); zinc oxide, such as "SnO-310" (manufactured by Sumitomo Osaka Cement Co., Ltd.), "SnO-350" (manufactured by Sumitomo Osaka Cement Co., Ltd.), and "SnO-410" (manufactured by Sumitomo Osaka Cement Co., Ltd.);

In a preferred embodiment of the technology disclosed herein, the difference between the refractive index of the thermally conductive filler contained in the adhesive layer and the refractive index of the adhesive layer is within ±0.04. With a configuration having an adhesive layer with such a small refractive index difference between the adhesive layer and the thermally conductive filler, an adhesive layer with high optical transparency can be easily achieved.

The refractive index of the adhesive layer and the refractive index of the thermally conductive filler may be in any magnitude relationship. In other words, the value obtained by subtracting the refractive index of the adhesive layer from the refractive index of the thermally conductive filler is preferably -0.04 or more and 0.04 or less. In some embodiments, the value obtained by subtracting the refractive index of the adhesive layer from the refractive index of the thermally conductive filler is preferably -0.02 or more and 0.04 or less, and may be 0 or more and 0.03 or less, 0 or more and 0.02 or less, or 0 or more and 0.015 or less. When the value obtained by subtracting the refractive index of the adhesive layer from the refractive index of the thermally conductive filler is in the above range, an adhesive sheet with high light transmittance is easily achieved.

The refractive index of the thermally conductive filler is not particularly limited. In some embodiments, the refractive index of the thermally conductive filler is preferably 1.70 or less, more preferably 1.65 or less, and even more preferably 1.60 or less. The lower limit of the refractive index of the thermally conductive filler is not particularly limited, but is usually 1.45 or more, preferably 1.50 or more, and even more preferably 1.55 or more.

The content of the thermally conductive filler in the adhesive layer is not particularly limited and can be set according to the thermal conductivity desired for the adhesive sheet. The content of the thermally conductive filler may be 5 parts by weight or more, 10 parts by weight or more, or 33 parts by weight or more, relative to 100 parts by weight of the base polymer. The content of the thermally conductive filler is preferably 50 parts by weight or more, more preferably 66 parts by weight or more, and even more preferably 100 parts by weight or more, relative to 100 parts by weight of the base polymer. The thermal conductivity of the adhesive layer tends to improve with an increase in the content of the thermally conductive filler. In some embodiments, the content of the thermally conductive filler may be 120 parts by weight or more, 150 parts by weight or more, or 185 parts by weight or more, relative to 100 parts by weight of the base polymer. In addition, from the viewpoint of suppressing a decrease in the optical transparency of the adhesive layer, or from the viewpoint of preventing a decrease in the surface smoothness of the adhesive layer and facilitating a good adhesion state with a member (e.g., an adherend), the content of the thermally conductive filler is usually 900 parts by weight or less relative to 100 parts by weight of the base polymer, preferably 400 parts by weight or less, more preferably 300 parts by weight or less or 250 parts by weight or less, and may be 200 parts by weight or less.

The average particle size of the thermally conductive filler is not particularly limited. The average particle size is usually 100 μm or less, preferably 50 μm or less, and may be 20 μm or less. When the average particle size is small, the surface smoothness of the adhesive layer improves, and the adhesion to the member (e.g., the adherend) tends to improve. In some embodiments, the average particle size of the thermally conductive filler may be 10 μm or less, 5 μm or less, or 3 μm or less. In addition, the average particle size of the filler may be, for example, 0.1 μm or more, 0.2 μm or more, or 0.5 μm or more. A not too small average particle size can be advantageous in terms of the handleability and dispersibility of the thermally conductive filler.

In some embodiments, the average particle size of the thermally conductive filler is preferably less than 0.5Ta relative to the thickness Ta of the adhesive layer. Here, the average particle size of the thermally conductive filler in this specification refers to the particle size (50% median diameter) at which the cumulative particle size on a weight basis is 50% in the particle size distribution obtained by measurement based on the sieving method, unless otherwise specified. If the average particle size of the thermally conductive filler is less than 50% of the thickness Ta of the adhesive layer, it can be said that 50% by weight or more of the thermally conductive filler contained in the adhesive layer has a particle size smaller than the thickness Ta of the adhesive layer. By having 50% by weight or more of the filler contained in the adhesive layer have a particle size smaller than the thickness Ta of the adhesive layer, there is a greater tendency for a good surface condition (e.g., smoothness) to be maintained on the adhesive surface. This is preferable from the viewpoint of improving thermal conductivity by improving adhesion to the adherend.

The adhesive sheet disclosed herein can be preferably implemented in an embodiment in which, in the particle size distribution obtained by the measurement based on the sieving method, 60% by weight or more of the thermally conductive filler contained in the adhesive layer has a particle size smaller than the thickness Ta of the adhesive layer (more preferably 0.7Ta, even more preferably 0.5Ta). The ratio of particles having a particle size smaller than the thickness Ta of the adhesive layer (more preferably 0.7Ta, even more preferably 0.5Ta) among the thermally conductive filler may be, for example, 70% by weight or more, 80% by weight or more, or 90% by weight or more. It is more preferable that substantially the entire amount of the thermally conductive filler contained in the adhesive layer has a particle size smaller than the thickness Ta of the adhesive layer (more preferably 0.7Ta, even more preferably 0.5Ta). Here, substantially the entire amount typically means 99% by weight or more and 100% by weight or less, for example 99.5% by weight or more and 100% by weight or less.

When the adhesive layer disclosed herein contains a thermally conductive filler, the thermal conductivity of the adhesive layer is not particularly limited, but is preferably 0.15 W/m·K or more. The higher the thermal conductivity, the easier it is to be placed between members for which heat dissipation or heat conduction is desired and to be suitably used for purposes such as heat dissipation and heat conduction of the members. The thermal conductivity is preferably 0.2 W/m·K or more, more preferably 0.25 W/m·K or more, and even more preferably 0.28 W/m·K or more, for example, 0.3 W/m·K or more, 0.31 W/m·K or more, or 0.32 W/m·K or more. There is no particular limit to the upper limit of the thermal conductivity of the adhesive layer. In consideration of the balance with other properties such as transparency, in some embodiments, the thermal conductivity of the adhesive layer may be, for example, 2.0 W/m·K or less, 1.5 W/m·K or less, 1.0 W/m·K or less, 0.8 W/m·K or less, 0.5 W/m·K or less, or less than 0.5 W/m·K. In some embodiments, the thermal conductivity of the adhesive layer may be 0.45 W/m·K or less, 0.40 W/m·K or less, or 0.35 W/m·K or less.

In this specification, the thermal conductivity of the adhesive layer or adhesive sheet refers to a value measured by a steady heat flow method. More specifically, the thermal conductivity of the adhesive layer or adhesive sheet can be measured by the method described in the examples below.

(Dispersant)
The adhesive composition for forming the adhesive layer may contain a dispersant as necessary to disperse the filler well in the adhesive composition. A pressure-sensitive adhesive layer having improved uniformity in thermal conductivity can be formed using the adhesive composition in which the filler is well dispersed.

As the dispersant, a known surfactant can be used. The surfactants include nonionic, anionic, cationic and amphoteric surfactants. The dispersant can be used alone or in combination of two or more.

An example of a preferred dispersant is a phosphate ester. For example, a monoester of phosphoric acid, a diester of phosphoric acid, a triester of phosphoric acid, a mixture of these, etc. can be used. Specific examples of phosphate esters include a monoester of phosphoric acid of polyoxyethylene alkyl ether, polyoxyethylene alkyl aryl ether, or polyoxyethylene aryl ether, a diester of phosphoric acid, a triester of phosphoric acid, and derivatives thereof. Suitable examples include a monoester of phosphoric acid of polyoxyethylene alkyl ether or polyoxyethylene alkyl aryl ether, and a diester of phosphoric acid of polyoxyethylene alkyl ether or polyoxyethylene alkyl aryl ether. The number of carbon atoms in the alkyl group in such a phosphate ester is, for example, 6 to 20, preferably 8 to 20, more preferably 10 to 18, and typically 12 to 16.

Commercially available products can be used as the above phosphate esters. Examples include "Plysurf A212E", "Plysurf A210G", "Plysurf A212C", and "Plysurf A215C" manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and "Phosphanol RE610", "Phosphanol RS710", and "Phosphanol RS610" manufactured by Toho Chemical Industry Co., Ltd.

The amount of dispersant used may be, for example, 0.01 to 25 parts by weight, and is usually 0.1 to 25 parts by weight, relative to 100 parts by weight of filler. From the viewpoint of preventing deterioration in the coatability of the adhesive composition and deterioration in surface smoothness due to poor dispersion of the filler, the amount of dispersant used relative to 100 parts by weight of filler is preferably 0.5 parts by weight or more, more preferably 1 part by weight or more, even more preferably 2 parts by weight or more, and may be 5 parts by weight or more. Furthermore, from the viewpoint of avoiding deterioration in performance such as adhesion due to excessive use of dispersant, the amount of dispersant used relative to 100 parts by weight of filler is preferably 20 parts by weight or less, more preferably 15 parts by weight or less, and may be 12 parts by weight or less, or 10 parts by weight or less.

The amount of dispersant used may be, for example, 0.01 to 25 parts by weight, and is usually 0.1 to 25 parts by weight, relative to 100 parts by weight of thermally conductive filler. From the viewpoint of preventing a decrease in the coatability of the adhesive composition and a decrease in the surface smoothness due to poor dispersion of the thermally conductive filler, the amount of dispersant used relative to 100 parts by weight of thermally conductive filler is preferably 0.15 parts by weight or more, more preferably 0.3 parts by weight or more, even more preferably 0.5 parts by weight or more, and may be 1 part by weight or more. Furthermore, from the viewpoint of avoiding a decrease in performance such as adhesion due to excessive use of the dispersant, the amount of dispersant used relative to 100 parts by weight of thermally conductive filler is preferably 20 parts by weight or less, more preferably 15 parts by weight or less, and may be 12 parts by weight or less, or 10 parts by weight or less.

In addition, the adhesive layer in the technology disclosed herein may contain, as necessary, known additives that can be used in adhesives, such as leveling agents, plasticizers, softeners, colorants (dyes, pigments, etc.), antistatic agents, antioxidants, UV absorbers, antioxidants, light stabilizers, preservatives, etc., to the extent that the effects of the present invention are not significantly impeded.

(Formation of Pressure-Sensitive Adhesive Layer)
The adhesive layer contained in the adhesive sheet disclosed herein may be a cured layer of the adhesive composition. That is, the adhesive layer may be formed by applying (for example, coating) the adhesive composition to a suitable surface and then appropriately carrying out a curing treatment. When two or more types of curing treatments (drying, crosslinking, polymerization, etc.) are performed, they can be performed simultaneously or in multiple stages. In an adhesive composition using a partial polymer of a monomer component (for example, an acrylic polymer syrup), typically, a final copolymerization reaction is performed as the curing treatment. That is, the partial polymer is subjected to a further copolymerization reaction to form a complete polymer. For example, in the case of a photocurable adhesive composition, light irradiation is performed. If necessary, a curing treatment such as crosslinking or drying may be performed. For example, if a photocurable adhesive composition needs to be dried, it is recommended to perform photocuring after drying. In an adhesive composition using a complete polymer, typically, a treatment such as drying (heat drying) or crosslinking is performed as the curing treatment as necessary.

The adhesive composition can be applied using a conventional coater such as a gravure roll coater, reverse roll coater, kiss roll coater, dip roll coater, bar coater, knife coater, or spray coater.

The thickness of the adhesive layer of the adhesive sheet disclosed herein is not particularly limited. From the viewpoint of improving thermal conductivity and light transmittance, the thickness of the adhesive layer is usually 600 μm or less, preferably 300 μm or less, more preferably 100 μm or less, and may be less than 100 μm, 80 μm or less, 70 μm or less, 60 μm or less, or 55 μm or less. From the viewpoint of improving the unevenness-following ability (which may also be understood as unevenness-absorbing ability) of the adhesive sheet, in some embodiments, the thickness of the adhesive layer may be, for example, 5 μm or more, 10 μm or more, 20 μm or more, 30 μm or more, or 40 μm or more.

In some embodiments, the adhesive layer may be formed from a solventless adhesive composition. Here, the term "solventless" refers to an adhesive composition having a solvent content of 5% by weight or less, typically 1% by weight or less. The solvent refers to a component that is not contained in the adhesive layer that is finally formed. Therefore, for example, unreacted monomers that may be contained in acrylic polymer syrup are excluded from the concept of the solvent. For example, a photocurable or hot melt adhesive composition can be used as the solventless adhesive composition. Among them, an adhesive layer formed from a photocurable (e.g., UV-curable) adhesive composition is preferred. The formation of an adhesive layer using a photocurable adhesive composition is often performed in a manner in which the adhesive composition is sandwiched between two sheets and cured by irradiating light while blocking air.

The technology disclosed herein makes it easy to obtain an adhesive layer that exhibits improved light transmittance (transparency). The transmittance of the adhesive layer disclosed herein is not particularly limited. For example, the transmittance of the adhesive layer is preferably 60% or more, more preferably 70% or more, and even more preferably 80% or more (e.g., 85% or more). There is no particular upper limit to the transmittance of the adhesive layer, but from the viewpoint of balancing with other properties such as thermal conductivity and adhesion, it is usually appropriate that the transmittance is 99% or less, and may be 95% or less, or may be 90% or less.

The transmittance of the adhesive sheet disclosed herein is not particularly limited. For example, the transmittance of such an adhesive sheet is preferably 50% or more, more preferably 65% or more, and even more preferably 80% or more (e.g., 85% or more). There is no particular upper limit to the transmittance of the adhesive sheet, but from the viewpoint of balancing with other properties such as thermal conductivity and adhesion, it is usually appropriate that the transmittance is 99% or less, and may be 95% or less, or may be 90% or less.

In this specification, the transmittance of the adhesive layer or adhesive sheet can be measured using a commercially available transmittance meter (for example, a high-speed integrating sphere spectroscopic transmittance meter, model "DOT-3", manufactured by Murakami Color Research Laboratory) at a temperature condition of 23°C and a measurement wavelength of 400 nm. More specifically, the transmittance of the adhesive layer or adhesive sheet can be measured by the method described in the examples below. The transmittance can be adjusted, for example, by selecting the composition and thickness of the adhesive layer.

The haze value of the adhesive layer disclosed herein is not particularly limited. For example, the haze value of the adhesive layer is usually 90% or less, preferably 80% or less, and more preferably 75% or less (e.g., 70% or less). In a preferred embodiment, the haze value of the adhesive layer is 60% or less, more preferably 50% or less, and even more preferably 40% or less. The lower limit of the haze value of the adhesive layer is not particularly limited. From the viewpoint of balancing with other properties (such as adhesive strength), the haze value of the adhesive layer is usually 0.5% or more, may be 10% or more, may be 20% or more, or may be 30% or more. For example, in an embodiment in which the adhesive layer contains a thermally conductive filler, the haze value of the adhesive layer may be 30% or more, may be 35% or more, may be 40% or more, may be 50% or more, may be 60% or more, or may be 65% or more.

The haze value of the adhesive sheet disclosed herein is not particularly limited. For example, the haze value of the adhesive sheet is usually 95% or less, preferably 85% or less, and more preferably 75% or less (e.g., 70% or less). In a preferred embodiment, the haze value of the adhesive sheet is 60% or less, more preferably 50% or less, and even more preferably 40% or less. The lower limit of the haze value of the adhesive sheet is not particularly limited. From the viewpoint of balancing with other properties (such as adhesive strength), the haze value of the adhesive sheet is usually 0.5% or more, and may be 10% or more, 20% or more, or 30% or more. For example, in an embodiment having an adhesive layer containing a thermally conductive filler, the haze value of the adhesive sheet may be 30% or more, 35% or more, 40% or more, 50% or more, 60% or more, or 65% or more.

Here, the "haze value" refers to the ratio of diffuse transmitted light to the total transmitted light when a measurement target is irradiated with visible light. It is also called the cloudiness value. The haze value can be expressed by the following formula.
Th(%)=Td/Tt×100
In the above formula, Th is the haze value (%), Td is the scattered light transmittance, and Tt is the total light transmittance. The haze value of the pressure-sensitive adhesive layer or pressure-sensitive adhesive sheet can be measured according to the method described in the Examples below. The haze value can be adjusted, for example, by selecting the composition, thickness, etc. of the pressure-sensitive adhesive layer.

<Applications>
According to this specification, a pressure-sensitive adhesive layer having a high refractive index while having excellent adhesive strength is provided. Therefore, the pressure-sensitive adhesive layer disclosed herein is suitable as a pressure-sensitive adhesive layer used in a pressure-sensitive adhesive sheet for optical applications. That is, a pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer disclosed herein is suitable as a pressure-sensitive adhesive sheet for optical applications. For example, the pressure-sensitive adhesive layer disclosed herein or a pressure-sensitive adhesive sheet having the pressure-sensitive adhesive layer is useful as a pressure-sensitive adhesive optical member using an optical member as a support. When an optical film is used as the optical member, the pressure-sensitive adhesive optical member is used as an optical film with a pressure-sensitive adhesive layer. As the optical film, a polarizing plate, a retardation plate, an optical compensation film, a brightness improvement film, a hard coat (HC) film, an anti-reflection film, an impact absorbing film, an antifouling film, a photochromic film, a light control film, a wavelength selective absorption film, a wavelength conversion film, and further a laminate of these can be used.

Examples of resin materials that can be used in the optical film include polyester resins such as polyethylene terephthalate and polyethylene naphthalate, cellulose resins such as triacetyl cellulose, acetate resins, polysulfone resins, polyethersulfone resins, polycarbonate resins, polyamide resins, polyimide resins, polyolefin resins, cyclic polyolefin resins (norbornene resins, etc.), acrylic resins, polyvinyl chloride resins, polyvinylidene chloride resins, polystyrene resins, polyvinyl alcohol resins, polyarylate resins, polyphenylene sulfide resins, and mixtures thereof. Among these, polyester resins, cellulose resins, polyimide resins, and polyethersulfone resins are preferred.

The adhesive sheet disclosed herein is not limited to the above-mentioned uses, and may be used for fixing, joining, forming, decorating, protecting, supporting, etc., of various portable devices (portable devices) in a manner that it is attached to the components that constitute the devices. Here, "portable" does not mean that it is merely portable, but that it has a level of portability that allows an individual (average adult) to carry it relatively easily. Examples of portable devices include mobile phones, smartphones, tablet computers, notebook computers, various wearable devices, digital cameras, digital video cameras, audio devices (portable music players, IC recorders, etc.), calculators (calculators, etc.), portable game devices, electronic dictionaries, electronic organizers, electronic books, in-vehicle information devices, portable radios, portable televisions, portable printers, portable scanners, portable modems, and other portable electronic devices, as well as mechanical wristwatches, pocket watches, flashlights, hand mirrors, etc. Examples of components that constitute the portable electronic devices include optical films and display panels used in image display devices such as liquid crystal displays and organic EL displays. The adhesive sheet disclosed herein can be attached to various components in automobiles, home appliances, etc., and can also be used favorably for applications such as fixing, joining, molding, decorating, protecting, and supporting the components.

In addition, when the adhesive layer disclosed herein is configured to include a thermally conductive filler, an adhesive sheet including the adhesive layer can also be used for heat dissipation of an adherend or heat transfer via the adhesive sheet. Furthermore, since the adhesive sheet disclosed herein has excellent transparency, the precision of device construction is likely to be improved for devices constructed using the adhesive sheet. Therefore, the adhesive sheet disclosed herein is suitable for heat dissipation of parts in precision instruments and small precision instruments that require high precision, or for heat transfer via the adhesive sheet, as well as for fixing, joining and supporting parts in precision instruments, etc.

Several examples of the present invention are described below, but it is not intended that the present invention be limited to those shown in these examples. In the following description, "parts" and "%" are by weight unless otherwise specified.

<Example 1>
(Preparation of Acrylic Polymer Syrup)
As monomer components, 50 parts of 2-ethylhexyl acrylate (2EHA), 50 parts of benzyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat #160"), 5 parts of N-vinyl-2-pyrrolidone (NVP), 2 parts of acrylic acid (AA) and 1 part of 4-hydroxybutyl acrylate (4HBA) were mixed with 0.05 parts of 1-hydroxycyclohexyl phenyl ketone (manufactured by BASF, product name "Irgacure 184") and 0.05 parts of 2,2-dimethoxy-1,2-diphenylethan-1-one (manufactured by BASF, product name "Irgacure 651") as photopolymerization initiators, and irradiated with ultraviolet light under a nitrogen atmosphere to polymerize until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measurement temperature 30°C) reached about 10 Pa·s, to produce acrylic polymer A in the form of an acrylic polymer syrup, which is a partial polymer with a polymerization rate of 5%.

(Preparation of Pressure-Sensitive Adhesive Composition)
To 30 parts of the acrylic polymer A (acrylic polymer syrup) prepared above, 30 parts of phenylphenol acrylate (ethoxylated o-phenylphenol acrylate, product name "A-LEN-10", manufactured by Shin-Nakamura Chemical Co., Ltd.) was added, and 0.03 parts of dipentaerythritol hexaacrylate (product name "KAYARAD DPHA", manufactured by Nippon Kayaku Co., Ltd.) as a polyfunctional monomer, 60 parts of aluminum hydroxide (product name "Aluminum Hydroxide B103", average particle size 7 μm, manufactured by Nippon Light Metal Co., Ltd.) as a thermally conductive filler, and 0.75 parts of a filler dispersant (product name "Plysurf A212E", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed, and the mixture was stirred at 1200 rpm for 5 minutes to prepare a pressure-sensitive adhesive composition A.

(Formation of Pressure-Sensitive Adhesive Layer)
Two types of release liners R1 and R2 were prepared, each having a polyester film with one side treated with a silicone release agent as the release surface. The release liner R1 was manufactured by Mitsubishi Plastics, Inc. under the trade name "Diafoil MRF" (thickness 38 μm). The release liner R2 was manufactured by Mitsubishi Plastics, Inc. under the trade name "Diafoil MRE" (thickness 38 μm).

The adhesive composition A prepared above was applied to the release surface of the release liner R1 to form a coating layer with a thickness of 50 μm. Next, the surface of the coating layer was covered with the release liner R2 so that the release surface was on the coating layer side, thereby blocking the coating layer from oxygen. This laminated sheet (having a laminated structure of release liner R1/coating layer/release liner R2) was irradiated with ultraviolet light at an illuminance of 3 mW/cm ² using a chemical light lamp manufactured by Toshiba Corporation for 360 seconds to harden the coating layer and form an adhesive layer. In this way, an adhesive sheet consisting of the adhesive layer was produced. The illuminance value was measured using an industrial UV checker (manufactured by Topcon Corporation, product name "UVR-T1", light receiving unit type UD-T36) with a peak sensitivity wavelength of about 350 nm.

<Example 2>
As monomer components, 45 parts of n-butyl acrylate (BA), 45 parts of benzyl acrylate (manufactured by Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat #160"), 10 parts of N-vinyl-2-pyrrolidone (NVP) and 2 parts of 4-hydroxybutyl acrylate (4HBA) were mixed with 0.05 parts of 1-hydroxycyclohexyl phenyl ketone (manufactured by BASF, product name "Irgacure 184") and 0.05 parts of 2,2-dimethoxy-1,2-diphenylethan-1-one (manufactured by BASF, product name "Irgacure 651") as photopolymerization initiators, and irradiated with ultraviolet light under a nitrogen atmosphere to polymerize until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measurement temperature 30°C) reached about 10 Pa·s, to produce acrylic polymer B in the form of an acrylic polymer syrup which is a partial polymer with a polymerization rate of 5%.

20 parts of the acrylic polymer B (acrylic polymer syrup) prepared above was mixed with 10 parts of phenylphenol acrylate (ethoxylated o-phenylphenol acrylate, product name "A-LEN-10", manufactured by Shin-Nakamura Chemical Co., Ltd.) as a polyfunctional monomer, 0.015 parts of dipentaerythritol hexaacrylate (product name "KAYARAD DPHA", manufactured by Nippon Kayaku Co., Ltd.), 30 parts of magnesium hydroxide (product name "Kisuma 5A", primary average particle size 1 μm, manufactured by Kyowa Chemical Industry Co., Ltd.) as a thermally conductive filler, and 0.76 parts of a filler dispersant (product name "Plysurf A212E", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) as a filler dispersant, and stirred at 1200 rpm for 5 minutes to prepare adhesive composition B.

The adhesive sheet of this example was produced in the same manner as in Example 1, except that adhesive composition B prepared above was used instead of adhesive composition A.

<Example 3>
To 16.2 parts of the acrylic polymer B (acrylic polymer syrup) prepared above, 13.8 parts of phenylphenol acrylate (ethoxylated o-phenylphenol acrylate, product name "A-LEN-10", manufactured by Shin-Nakamura Chemical Co., Ltd.) was added, and further, 0.015 parts of dipentaerythritol hexaacrylate (product name "KAYARAD DPHA", manufactured by Nippon Kayaku Co., Ltd.) as a polyfunctional monomer, 30 parts of aluminum hydroxide (product name "Aluminum Hydroxide BF013", primary average particle size 1 μm, manufactured by Nippon Light Metal Co., Ltd.) as a thermally conductive filler, and 0.38 parts of a filler dispersant (product name "Plysurf A212E", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed and stirred at 1200 rpm for 5 minutes to prepare a pressure-sensitive adhesive composition C.

The adhesive sheet of this example was produced in the same manner as in Example 1, except that adhesive composition C prepared above was used instead of adhesive composition A.

<Example 4>
To 40 parts of the acrylic polymer B (acrylic polymer syrup) prepared above, 20 parts of phenylphenol acrylate (ethoxylated o-phenylphenol acrylate, product name "A-LEN-10", manufactured by Shin-Nakamura Chemical Co., Ltd.) was added, and further, 0.03 parts of dipentaerythritol hexaacrylate (product name "KAYARAD DPHA", manufactured by Nippon Kayaku Co., Ltd.) as a polyfunctional monomer, 30 parts of magnesium hydroxide (product name "Kisuma 5A", primary average particle size 1 μm, manufactured by Kyowa Chemical Industry Co., Ltd.) and 30 parts of magnesium hydroxide (product name "Magnesium Hydroxide #300", primary average particle size 5 μm, manufactured by Konoshima Chemical Co., Ltd.) as a thermally conductive filler, and 1.52 parts of a filler dispersant (product name "Plysurf A212E", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed and stirred at 1200 rpm for 5 minutes to prepare a pressure-sensitive adhesive composition D.

The adhesive sheet of this example was produced in the same manner as in Example 1, except that adhesive composition D prepared above was used instead of adhesive composition A.

<Example 5>
80 parts of 2-ethylhexyl acrylate (2EHA), 12 parts of 2-methoxyethyl acrylate (MEA), 7 parts of N-vinyl-2-pyrrolidone (NVP) and 1 part of N-(2-hydroxyethyl)acrylamide (HEAA) as monomer components were mixed with 0.05 parts of 1-hydroxycyclohexyl phenyl ketone (manufactured by BASF, trade name "Irgacure 184") and 0.05 parts of 2,2-dimethoxy-1,2-diphenylethan-1-one (manufactured by BASF, trade name "Irgacure 651") as photopolymerization initiators, and irradiated with ultraviolet light under a nitrogen atmosphere to polymerize until the viscosity (BH viscometer, No. 5 rotor, 10 rpm, measurement temperature 30°C) reached about 20 Pa·s, to produce a partial polymer with a polymerization rate of 5%. To 70 parts of the prepared partial polymer, 30 parts of 2-ethylhexyl acrylate (2EHA) was added as a diluent monomer to prepare an acrylic polymer C in the form of an acrylic polymer syrup.

42 parts of the acrylic polymer C (acrylic polymer syrup) prepared above, 0.05 parts of dipentaerythritol hexaacrylate (trade name "KAYARAD DPHA", manufactured by Nippon Kayaku Co., Ltd.) as a polyfunctional monomer, 120 parts of aluminum hydroxide (trade name "Aluminum Hydroxide B103", manufactured by Nippon Light Metal Co., Ltd., average particle size 7 μm) as a thermally conductive filler, and 0.75 parts of a filler dispersant (trade name "Plysurf A212E", manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) were mixed and stirred at 1200 rpm for 5 minutes to prepare adhesive composition E.

The adhesive sheet of this example was produced in the same manner as in Example 1, except that adhesive composition E prepared above was used instead of adhesive composition A.

<Example 6>
50 parts of a bifunctional acrylate compound having a fluorene skeleton (manufactured by Osaka Gas Chemicals Co., Ltd., product name "OGSOL EA-0300"), 10 parts of a phenylphenol acrylate (manufactured by Shin-Nakamura Chemical Co., Ltd., ethoxylated o-phenylphenol acrylate, product name "A-LEN-10"), 60 parts of aluminum hydroxide (manufactured by Nippon Light Metal Co., Ltd., product name "Aluminum Hydroxide B103", average particle size 7 μm) as a thermally conductive filler, and 0.75 parts of a filler dispersant (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name "Plysurf A212E") were mixed and stirred at 1200 rpm for 5 minutes to prepare a pressure-sensitive adhesive composition F.

The adhesive sheet of this example was produced in the same manner as in Example 1, except that adhesive composition F prepared above was used instead of adhesive composition A.

The glass transition temperature (Tg) of the acrylic polymer (base polymer) contained in the adhesive for each example is summarized in Table 1.

<Refractive index>
The refractive index of the adhesive sheet (i.e., adhesive layer) according to each example was measured using a multi-wavelength Abbe refractometer (manufactured by ATAGO, model number "DR-M2") under measurement conditions of a wavelength of 589 nm and 23°C (the same as in the measurement of the refractive index of the thermally conductive filler below), and the obtained value is shown in the "Adhesive Layer" column of Table 1. In addition, the refractive index of aluminum hydroxide and/or magnesium hydroxide used as the thermally conductive filler used to prepare the adhesive sheet according to each example was measured, and the obtained value is shown in the "Thermal Conductive Filler" column of Table 1. In addition, for the adhesive sheet according to each example, a value obtained by subtracting the refractive index of the adhesive layer from the refractive index of the thermally conductive filler measured above was calculated, and the obtained value is shown in the "Refractive Index Difference" column of Table 1.

<Measurement of Haze Value>
The release liner R1 was peeled off from the pressure-sensitive adhesive sheet according to each example, and the exposed adhesive surface was attached to one side of an alkali glass having a haze of 0.1%, and then heated at 80° C. for 5 minutes to fully develop adhesive strength to the alkali glass. The release liner R2 was then peeled off from the pressure-sensitive adhesive sheet to prepare a test piece, and the haze value was measured using a haze meter (MR-100, manufactured by Murakami Color Research Laboratory). In the measurement, the alkali glass to which the pressure-sensitive adhesive sheet was attached was positioned so that the pressure-sensitive adhesive sheet was on the light source side. Since the haze value of the alkali glass is 0.1%, the haze value of the pressure-sensitive adhesive sheet (adhesive layer) was determined by subtracting 0.1% from the measured value, and the obtained value is shown in the "Haze Value" column in Table 1.

<Measurement of adhesive strength to SUS>
The adhesive sheet of each example was cut together with the release liner into a width of 20 mm to prepare a test piece. A SUS plate (SUS304BA plate) cleaned with toluene was used as an adherend, and the adhesive strength (adhesive strength after heating) was measured according to the following procedure.
(Measurement of Adhesive Strength)
Under a standard environment of 23°C and 50% RH, the release liner covering the adhesive surface of each test piece was peeled off, and the exposed adhesive surface was pressed against the adherend by rolling a 2 kg roller back and forth once. The test piece thus pressed against the adherend was heated at 80°C for 5 minutes, and then left for 30 minutes under the above-mentioned standard environment, and then the 180° peel adhesion (resistance to the above-mentioned tension) was measured using a universal tension and compression tester (apparatus name "Tension and compression tester, TCM-1kNB" manufactured by Minebea Co., Ltd.) in accordance with JIS Z0237 under the conditions of a peel angle of 180° and a tensile speed of 300 mm/min. The measurement was performed three times, and the average value of the measurements was shown in the "Adhesive Strength" column of Table 1 as the adhesive strength (adhesive strength after heating).

<Measurement of thermal conductivity>
The thermal conductivity in the thickness direction of the pressure-sensitive adhesive sheet (i.e., the pressure-sensitive adhesive layer) according to each example was measured using a thermal property evaluation device shown in Figures 3(a) and 3(b). Figure 3(a) is a front schematic view of the thermal property evaluation device, and Figure 3(b) is a side schematic view of the thermal property evaluation device. Note that the release liners R1 and R2 were removed during the measurement.

Specifically, an adhesive sheet S (square shape, 20 mm long and 20 mm wide) was sandwiched between a pair of blocks (sometimes called "rods") L made of aluminum (A5052, thermal conductivity: 140 W/mK) formed into a cube with one side measuring 20 mm, and the pair of blocks L was brought into close contact with the adhesive sheet S. The pair of blocks L were then arranged vertically between a heating element (heater block) H and a heat sink (cooling base plate configured so that cooling water circulates inside) C. Specifically, the heating element H was arranged on top of the upper block L, and the heat sink C was arranged below the lower block L.
At this time, the pair of blocks L in close contact with the adhesive sheet S are positioned between a pair of pressure adjustment screws J penetrating the heating element H and the heat dissipation element C. A load cell R is disposed between the pressure adjustment screws J and the heating element H, and is configured to measure the pressure when the pressure adjustment screws J are tightened. This pressure was used as the pressure applied to the adhesive sheet S. Specifically, in this test, the pressure adjustment screws J were tightened so that the pressure applied to the adhesive sheet S was 25 N/cm ² (250 kPa).
In addition, three probes P (diameter 1 mm) of a contact type displacement meter were installed so as to penetrate the lower block L and the adhesive sheet S from the heat sink C side. At this time, the upper end of the probe P was in contact with the lower surface of the upper block L, and was configured to be able to measure the distance between the upper and lower blocks L (the thickness of the adhesive sheet S).
Temperature sensors D were attached to the heating element H and the upper and lower blocks L. Specifically, the temperature sensor D was attached to one location on the heating element H. In addition, temperature sensors D were attached to five locations on each block L at intervals of 5 mm in the vertical direction.
In the measurement, first, pressure was applied to the adhesive sheet S by tightening the pressure adjusting screw J, the temperature of the heating element H was set to 80° C., and cooling water at 20° C. was circulated through the heat sink C.
After the temperatures of the heating element H and the upper and lower blocks L stabilized, the temperatures of the upper and lower blocks L were measured by each temperature sensor D, and the heat flux passing through the adhesive sheet S was calculated from the thermal conductivity [W/m·K] and temperature gradient of the upper and lower blocks L, and the temperature of the interface between the upper and lower blocks L and the adhesive sheet S was calculated. Using these, the thermal conductivity [W/m·K] at the above pressure was calculated using the following thermal conductivity equation (Fourier's law). The obtained values are shown in the "thermal conductivity" column of Table 1.

Q = -λ gradT
However, in the above formula,
Q: Heat flow rate per unit area gradT: Temperature gradient λ: Thermal conductivity

As is clear from the results shown in Table 1, the adhesive sheets of Examples 1 to 4, in which the refractive index of the adhesive layer is 1.54 or higher and the glass transition temperature of the acrylic polymer (base polymer) of the adhesive contained in the adhesive layer is 5°C or lower, have a clearly lower haze value of the adhesive sheet (adhesive layer) and have improved light transmittance, compared to the adhesive sheet of Example 5, in which the refractive index of the adhesive layer is less than 1.54. In addition, it was confirmed that the adhesive sheets of Examples 1 to 4 have a clearly higher adhesive strength than the adhesive sheet of Example 6, in which the glass transition temperature of the acrylic polymer contained in the adhesive is higher than 5°C.

In addition, it was confirmed that the adhesive sheets of Examples 1 to 4 had low haze values and high light transmittance, even though they were configured to contain aluminum hydroxide or magnesium hydroxide as a thermally conductive filler such that the thermal conductivity of the adhesive sheet was 0.32 W/m·K or higher.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples given above.

10 Adhesive sheet (double-sided adhesive sheet)
12 Adhesive layer 12A First adhesive surface 12B Second adhesive surface 14 Release liner 16 Release liner 20 Adhesive sheet (double-sided adhesive sheet)
22 Support substrate 22A First surface 22B Second surface 24 First adhesive layer 24A Surface of first adhesive layer (first adhesive surface)
26 Second adhesive layer 26A Surface of second adhesive layer (second adhesive surface)
28 Release liner 29 Release liner 100 Pressure-sensitive adhesive sheet with release liner 200 Pressure-sensitive adhesive sheet with release liner

Claims (5)

A pressure-sensitive adhesive layer comprising a pressure-sensitive adhesive obtained using a pressure-sensitive adhesive composition comprising a base polymer,
The pressure-sensitive adhesive layer has a refractive index of 1.54 or more,
the base polymer is an acrylic polymer;
The acrylic polymer is a polymer of monomer components including one or both of n-butyl acrylate and 2-ethylhexyl acrylate, and a (meth)acrylate having an aromatic ring;
the total amount of the n-butyl acrylate and the 2-ethylhexyl acrylate accounts for 15% by weight or more based on the total amount of the monomer components;
the proportion of the (meth)acrylate having an aromatic ring in the total amount of the monomer components is 50% by weight or more;
The (meth)acrylate having an aromatic ring includes benzyl (meth)acrylate and optionally ethoxylated phenylphenol (meth)acrylate,
The glass transition temperature of the base polymer is 5° C. or less,
The adhesive layer further comprises a thermally conductive filler . The pressure-sensitive adhesive layer according to claim 1 , wherein the total amount of the n -butyl acrylate and the 2-ethylhexyl acrylate is 20% by weight or more based on the total amount of the monomer components. The pressure-sensitive adhesive layer according to claim 1 , comprising a hydrated metal compound as the thermally conductive filler. The pressure-sensitive adhesive layer according to any one of claims 1 to 3 , wherein a refractive index difference between the pressure-sensitive adhesive layer and the thermally conductive filler is within ±0.04. A pressure-sensitive adhesive sheet comprising the pressure-sensitive adhesive layer according to any one of claims 1 to 4 .

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